The following two equilibria exist simultaneously in a closed vessel:PCl5(g)⇌PCl3(g)+Cl2(g)COCl2(g)⇌CO(g)+Cl2(g)If some CO is added into the vessel, after the equilibrium is attained again, concentration of

On adding CO, equilibrium (i) will shift backward. Hence concentration of Cl2 will decrease. Decrease in concentration of Cl2 will shift equilibrium (i) in the forward direction, i.e., more PCl5, will dissociate. Hence, concentration of PCl5, will decrease.